1. Field of the Invention
This invention relates to a power transmission system for the transmission of power or the transmission of movement from a first device to a second device. In general, such power transmission systems operate by means of belts, i.e. endless flexible belts which operate by mechanical adherance, or friction. In particular, such belts can be toothed belts, also called synchronous belts, as used, for example, on internal combustion engines for the synchronous propulsion of cam shafts at the cylinder head.
The vehicle engine generally encloses the synchronous toothed belt in a housing which is generally not very well vented, but which housing can theoretically be quite dry. When used on an internal combustion engine of a vehicle, such belts can typically be exposed to detrimental conditions such as sprays of high temperature oil. Further, because of the lack of ventilation for such a housing, the belt, as well as being exposed to possible oil sprays, is also exposed to the high temperatures produced by the engine in the form of thermal radiation which emanates from the walls of the crankcase.
2. Background Information
Such endless transmission belts are typically composed of a flexible armature in combination with at least one elastomer. On a first side of the belt, there can be a plurality of teeth formed by a first elastomer, and preferably covered by a protective fabric, while the opposite side of the belt can comprise an elastomer compound that can be identical to or different from the elastomer compound used for the teeth. On this opposite side of the belt, the elastomer compound essentially forms the back-side of the belt and serves to protect the flexible armature. This back-side can be smooth, can have longitudinal corrugations for a friction drive, or can have identical or different teeth for engagement on matching pulleys.
The problem of creep during operation, which has long been a source of problems and the failure of such belts, has been solved by the utilization of steel, glass or aramid fibers as the flexible armature. Thus, the useful life of synchronous belts has, for the most part, become a direct function of the mechanical strength and thermal resistance of the elastomer base compounds used in the composition of the teeth and of the back of the belts.
Because of the problems associated with internal combustion engines, such transmission belts generally needed to be made from an elastomer which was resistant to repeated bending stresses, was resistant to ozone and oil, and had a high-temperature resistance up to about 100 degrees C. For these purposes, polychloroprene base compounds generally provided satisfactory results. However, given the increased severity of the conditions in which such belts currently are used, and in particular the increase in temperatures under the hood, the previously used polychloroprene compounds are currently insufficient. In this regard, given the increasing severity of the requirements set by designers, manufacturers of automobile equipment have particularly devoted their research efforts to high-performance elastomer compounds. Such new high-performance elastomer compounds have been made possible by the development of new polymers.
In an attempt to meet the increased requirements for transmission belts, French Patent No. 2 334 021 to Pirelli proposes the use of epichlorohydrin matrix compounds, and French Patent No. 2 628 678 to Tosch Corp. proposes the use of chlorosulfonated ethylenealpha-olefin copolymer (CSM) matrix compounds for constructing the belts. It should be noted, however, that although the belts realized with these elastomers exhibit a positive increase of operational characteristics at higher temperatures, such belts retain a sensitivity to oils which thereby reduces their performance in case of accidental contact with any oil.
To remedy the disadvantage of insufficient resistance to hot oils, while maintaining a satisfactory level of useful life, other embodiments have been proposed with compounds of hydrogenated butadiene-acrylonitrile copolymer (HNBR) matrix, which is a highly saturated nitrile.
Belts constructed with the same hydrogenated butadiene-acrylonitrile copolymer (HNBR) matrix elastomer compound on the back and in the teeth, i.e. the entire belt being homogeneously constructed with the HNBR elastomer, are disclosed in International Publication No. WO 89/12076 to Nippon Zeon and Japanese Patent No. 01-269743 to Bando Chemical. Nevertheless, the improvement of the resistance to oils is achieved at the expense of an increased cost of making such belts, due to the higher cost of the base elastomer.
Japanese Patent No. 64-588311, also to Bando, discloses an improvement in performance, without an excessive cost increase. This improvement is the result of the combination of an alkylated chlorosulfonated ethylene-alpha-olefin copolymer (ACSM), forming the back and the teeth of the belt, with a fabric covering on the teeth, which fabric covering is impregnated with a hydrogenated butadiene-acrylonitrile copolymer (HNBR) matrix elastomer compound. A major problem with such a belt is the adherence of the fabric coated with a mixture of hydrogenated butadiene-acrylonitrile copolymer (HNBR) to the mass of the alkylated chlorosulfonated ethylene-alpha-olefin copolymer (ACSM). At high stresses, the weak points in the structure provide possible areas for the advance of moisture or oil into the belt via the cut flanks of the belt and the seams between elements of the fabric covering of the teeth.
The interface between the fabric and the alkylated chlorosulfonated ethylene-alpha-olefin copolymer (ACSM) matrix compound forming the teeth, is essentially the most highly-stressed point of the belt, and is subjected, in particular, to bending stresses as the base of the teeth.
Hairline cracks in the elastomer compound essentially signal the end of the useful life of the belt. Such hairline cracks have a tendency to appear first at this interface at the base of the teeth, and this interface area is typically the point which is the least well protected against oil impregnations.